Deficit irrigation of potato

C.C. Shock and E.B.G. Feibert,
Malheur Experiment Station,
Oregon State University,
Oregon, United States of America

Summary

Potato (Solanum tuberosum L.) can respond to water stress with yield reductions and loss of tuber grade. The economic opportunities to practise deficit irrigation are more limited for potato than for some other crops. Four potato varieties were grown under four, season-long, sprinkler irrigation treatments during three successive years (1992-1994) on a silt loam soil in eastern Oregon, United States of America. The check treatment was irrigated when soil water potential (SWP) at the 0.2-m depth reached -60 kPa. This treatment received at most the accumulated evapotranspiration (ETc) to avoid exceeding the water holding capacity of the top 0.3 m of soil. The three deficit irrigation treatments were irrigated when SWP at the 0.2-m depth reached -80 kPa and had the following percent of the accumulated ETc applied at each irrigation: (i) 100 percent, (ii) 70 percent, and (iii) 50 percent, with 70 percent during tuber bulking. Based on regression of applied water over three years, potatoes lost both total yield and grade when irrigations were reduced. Based on regression of applied water reductions in irrigation, gross revenues declined more than production costs, resulting in reduced profits. The results of this case study suggest that deficit irrigation of potatoes in the semi-arid environment of eastern Oregon would not be a viable management tool because the small financial benefits would not offset the high risks of reduced yields and profits from the reduced water applications. Results from eastern Oregon are compared with those obtained elsewhere.

Political constraints, rising costs, and groundwater scarcities are resulting in less water being available for agriculture. In some areas, groundwater supplies are being exhausted. Competition for water supplies is a worldwide phenomenon. In the Pacific northwest of the United States, political pressures are growing to reallocate water from irrigation to provide instream flows for preserving native fish populations, to provide for water and power needs of growing urban areas, and to reduce non-point source pollution of groundwater and surface water. Deficit irrigation may be one approach to address these issues.

Deficit irrigation is a strategy which allows a crop to sustain some degree of water deficit in order to reduce irrigation costs and potentially increase revenues. English and Raja (1996) described three deficit irrigation case studies in which the reductions in irrigation costs were greater than the reductions in revenue due to reduced yields. Deficit irrigation can lead, in principle, to increased profits where water costs are high or where water supplies are limited. In these case studies, crop value was associated closely with yield, and crop grade and marketability were not germane. Under these circumstances, deficit irrigation can be a practical choice for growers.

Deficit irrigation has proved successful with a number of crops in various parts of the world. These crops are relatively resistant to water stress, or they can avoid stress by deep rooting, allowing access to soil moisture lower in the soil profile. However, deficit irrigation of potatoes may be difficult to manage because reductions in tuber yield and quality can result from even brief periods of water stress following tuber set (Eldredge et al., 1992; Lynch et al., 1995; Shock et al., 1993; Wright and Stark, 1990). For cv. Russet Burbank, which is predominant in the Pacific northwest of the United States of America, short-duration shortages in water supply during early tuber bulking induced losses in tuber grade (Robins and Domingo, 1956; Salter and Goode, 1967; Thompson and Kelly, 1957) and internal quality directly related to market value (Eldredge et al., 1996). However, in some circumstances, potatoes can tolerate limited deficit irrigation before tuber set without significant reductions in external and internal tuber quality (Shock et al., 1992). Potato varieties differ in tolerance to water stress (Jefferies and MacKerron, 1993a, b; Lynch and Tai, 1989; Martin and Miller, 1983; Miller and Martin, 1987a, b). The adoption of new potato cultivars by growers and processors makes it desirable to re-examine deficit irrigation.

The advent of more efficient irrigation methods allied with the use of soil moisture monitoring devices can make deficit irrigation of potatoes more manageable. Sprinkler irrigation and subsurface drip irrigation (SDI) permit more precise control of the amount of water applied than does furrow irrigation, allowing accurate management of crop rootzone soil moisture. Irrigation scheduling with estimated crop evapotranspiration (ETc) and a target soil water potential (SWP) level can provide the feedback for managing irrigations. Careful irrigation scheduling has resulted in optimum potato yield and quality. For silt loam, the soil water potential in the top 0.3 m should remain wetter than -60 kPa (Eldredge et al.,1996).

The objectives of this research were: (i) to determine potato response to mild, season-long precision deficit irrigation by partial ETc replacement at a SWP of -80 kPa; (ii) to compare the responses of several major commercial varieties to deficit irrigation; and (iii) to evaluate the potential for deficit irrigation to improve the economic efficiency of potato production.

Materials and methods

Deficit irrigation trials at Oregon State University's Malheur Experiment Station, Ontario, Oregon, United States of America, were conducted in three successive years on an owyhee silt loam (coarse-silty, mixed, mesic, xerollic camborthid). As the cultural practices are described elsewhere (Shock et al., 1998), this section includes only the details related to the irrigation treatments, irrigation scheduling, and the evaluation of the potato crops.

In the experimental design, irrigation treatments were the main plots, replicated five times. The varieties were split plots within the main plots. The varieties were: Russet Burbank, Shepody, Frontier Russet, and Ranger Russet. Irrigation treatments were arranged in randomzied complete blocks and consisted of an adequately irrigated check and three progressively drier deficit irrigation treatments (Table 1). The control treatment was irrigated when the soil water potential at 0.2-m depth reached -60 kPa and received no more water than the accumulated ETc since the previous irrigation. The deficit irrigation treatments were irrigated when the SWP at 0.2 m reached -80 kPa and had a percentage of the accumulated ETc since the last irrigation applied at each irrigation: i) 100 percent; ii) 70 percent; and iii) 50 percent until tuber set, then 70 percent for six weeks, and 50 percent thereafter.

To reduce the risk of water losses through leaching, each irrigation was limited to avoid exceeding the water holding capacity of the soil to a depth of 0.3 m. For the control treatment, individual water applications did not exceed 30 mm, and for the plots irrigated at -80 kPa with 100 percent ETc replaced, individual water applications did not exceed 35 mm. The level of -80 kPa was chosen as it was the SWP at which a single episode of water stress, during tuber bulking, had been previously reduced Russet Burbank tuber grade and quality at the experimental site (Eldredge et al.,1996).

Plots were 13 rows wide (12 m) and 12 m long. Each plot was irrigated using sprinkler heads adjusted to cover a 90° angle at each corner of the plot. The water application rate was 10 mm/h and the coefficient of uniformity for the sprinkler system, calculated according to Christiansen (1942), was 86 percent. All plots with the same treatment were irrigated when the average SWP of the sensors for those plots reached the treatment threshold value. Each year, the irrigations were initiated no earlier than one week before tuber set.

Soil water potential was measured in each plot by two granular matrix sensors (GMSs; Watermark Soil Moisture Sensors, Model 200SS, Irrometer Co., Riverside, California, United States of America) centred at the 0.2-m depth and two GMSs centred at the 0.5-m depth. The four GMSs in each plot were offset 0.15 m from the hill centre (Stieber and Shock, 1995). Sensor readings had been calibrated against tensiometer measurements of SWP (Eldredge et al., 1993). The GMSs were read at 0800 hours daily starting a few days before tuber set each year. Potato ETc was estimated daily and recorded from crop emergence until the final irrigation, using an AgriMet (United States Bureau of Reclamation, Boise, Idaho, United States of America) weather station at the Malheur Experiment Station and a modified Penman equation (Wright, 1982). Treatments could be irrigated daily, as needed, because sensor readings and Etc calculations were available daily.

Tubers were harvested and graded by market class (U.S. No. 1 and U.S. No. 2) and size (113-170 g, 170-283 g, and >283 g). They were graded as U.S. No. 2 where any of the following conditions existed: growth cracks, bottleneck shape, abnormally curved shape, or two or more knobs.

Tuber specific gravity and stem-end fry colour were determined (Shock et al., 1994). Monetary values for the crops were calculated according to a 1996 potato growing and sales contract for processing potatoes (ORE-IDA Foods, Inc., Boise, Idaho, United States of America). Potato production costs were calculated from data prepared by Malheur County Extension (Oregon State University, Ontario, Oregon, United States of America) and were considered the same for all treatments except for harvest costs, which were calculated per unit of total yield. Irrigation costs were calculated from data prepared by Patterson et al. (1996) and were considered the same for all treatments except for pump power costs, calculated per millimetre of water applied. Total yields and U.S. No. 1 yields and net profits averaged across varieties were regressed against applied water plus rainfall for the three years.

Results

Water applications over time for all treatments were close to, and less than, the target ETc values each year (Table 1 and Figure 1). Preci-pitation during the tuber bulking period was 46, 57 and 7 mm for 1992, 1993 and 1994, respectively. The number of days with SWP at 0.2-m depth below -60 kPa increased with the change in the irrigation criterion from -60 to -80 kPa and with the decreases in applied water (Table 1). The accumulated growing degree days (10-30° C) during the tuber bulking period were 931, 695 and 946 for 1992, 1993 and 1994, respectively.

Table 1Irrigation treatments for potato

Treatment

1992

1993

1994

Irrigationcriterion(kPa)

Irrigation amount(% ETc )

Total water applied(mm)

AverageSWPa (kPa)

Time, w/ SWP<-60 kPa (d)

Total water applied(mm)

Ave.SWP (kPa)

Time w/ SWP<-60 kPa(d)

Total water applied(mm)

Ave.SWP (kPa)

Time w/
SWP<-60 kPa(d)

-60
-80
-80
-80

100
100
70
50,70,50b

589
566
411
368

-50
-64
-58
-72

11
25
35
44

466
255
259
259

-30
-41
-51
-63

3
12
21
36

544
380
356
327

-37
-54
-59
-60

4
26
26
31

LSD0.05

46

22

18

39

14

12

70

17

14

a Average daily, 0800 hours measurements at 0.2-m depth, from five plots, recorded a few days before tuber set through to 7 September each yearb 50% of accumulated ETC replaced until tuber set, then 70% of ETC replaced for six weeks, then 50% of ETC replaced until last irrigation
ETc estimates: 1992 - 66 mm; 1993 - 491 mm; 1994 - 622 mm.

Figure 1Cumulative ETc and water applied plus rainfall for potatoes submitted to four irrigation treatments, 1994

Treatment 1 was irrigated at -60 kPa and had a target of 100% of ETc applied.Treatments 2, 3 and 4 were irrigated at -80 kPa and had targets of 100%, 70%, and <70% of ETc applied, respectively; data for 1992 and 1993 were similar.

Tuber yields in the well-irrigated treatments of this trial averaged 57 Mg/ha, while Malheur County growers had an average yield of 46 Mg/ha over the same years with the cultivars Shepody and Russet Burbank. Reductions in total yield due to the progressive deficit irrigation treatments averaged 6.7, 10 and 14 percent with corresponding water savings of 25, 36 and 40 percent. Total yield and U.S. No. 1 yield both increased with increases in water supply in each of the three years (Figure 2).

Figure 2Effect of irrigation plus precipitation on potato tuber yield for three years averaged over four varieties

The irrigation x cultivar interaction was significant only in 1992 for total and U.S. No. 2 yields. In 1992, U.S. No. 2 yield of Russet Burbank increased with deficit irrigation whereas total yield was insensitive. In contrast, U.S. No. 2 yields for Frontier Russet, Ranger Russet, and Shepody were insensitive to deficit irrigation, whereas total yields declined.

Deficit irrigation had small effects on tuber stem-end fry colour in 1992 and 1993, and was associated with reduced tuber specific gravity only in 1994. The market value of the crop includes considerations of marketable yield, tuber size and grade, fry colour, and specific gravity. Based on the prevailing market contract, estimated profit to the grower decreased on average by 32, 41 and 68 percent with corresponding average water savings of 25, 36 and 40 percent.

Discussion

Potato water requirements

Potato ETc averaged 593 mm over the three years of the study. Potato ETc requirements are well established and are based on weather data, the timing of the stages of plant development, canopy coverage, and crop coefficients during development (Wright and Stark, 1990). They range broadly from less than 300 to 700 mm, depending on the environment, the year, and rate of crop growth.

Yield responses to irrigation deficits

Yield and grade responded linearly to applied water. In arid regions, studies have shown that potato yield responds linearly to applied water where irrigation plus rainfall is less than or equal to ETc (Hane and Pumphrey, 1984; Hegney and Hoffman, 1997; Martin et al., 1992; Shalhevet et al., 1983). Losses in potato yield and grade in response to deficit irrigation were in agreement with previous observations, e.g. Eldredge et al. (1992) and Stark and McCann (1992).

Tuber grade responses to irrigation deficits

External tuber defects that cause loss of grade are consistent with water stress during early formation and bulking of the tubers (Robins and Domingo, 1956; Salter and Goode, 1967; Thompson and Kelly, 1957).

Tuber internal quality responses to water stress

Short-term deficit irrigation intensities (driest SWP experienced) in this study were within the ranges of SWP for a silt loam that resulted in dark stem-end fry colour and loss in tuber specific gravity in previous work (Eldredge et al., 1996). The lack of consistent stem-end fry colour response or loss in tuber specific gravity to the season-long deficit irrigation in this study indicates that the potato plants may have become drought hardened in the manner hypothesized by van Loon (1981). The use of sensors for SWP feedback allowed the regulation of stress, such that it reoccurred at the same level throughout the growing season.

In contrast, well-watered potato plants, subjected to irrigation deficits after tuber initiation during the middle of the growing season, produced tubers with reduced specific gravity (Hang and Miller, 1986; Miller and Martin, 1987b). Miller and Martin (1987a) found that the specific gravity of Russet Burbank fell following deficit irrigation at 80 percent of ETc on a sandy soil. Stark and McCann (1992) reported reduced specific gravity and darker stem-end fry colour for Russet Burbank subjected to deficit irrigation at 80 percent of ETc on a silt loam soil. In the present study, irrigation management maintained rootzone SWP higher than -80 kPa, thus attenuating the intensity of water stress resulting from the deficit irrigation treatments. The aforementioned studies, despite using daily irrigations, did not use SWP feedback for irrigation scheduling.

Differential response of varieties to deficit irrigation

The variety x irrigation-treatment interactions were not consistently strong in the present study. Other authors have found strong potato-genotype x water-stress interactions (Jefferies and MacKerron, 1993b).

Economic outcome

Deficit irrigation reduced gross revenues more than production costs (Shock et al., 1998). Reductions in water applied resulted in small decreases in irrigation costs, because only electrical power for the pumping was saved. Water costs independent of pumping did not diminish with decreased irrigation because the district charged a fixed fee per hectare of water. Cost reductions with deficit irrigation would be greater than in the present study if the pumping lift were high or the water more costly. Over the three years, profits rose with increases in applied water. These results are complementary to those of Stark and McCann (1992), who observed declines in yield, grade, specific gravity, and fry colour for processing potatoes grown at Kimberly, Idaho, United States of America, with deficit irrigation.

In this study, the environmental benefits of the well watered control treatment were significant, with 10 percent less water applied than full estimated ETc and with a low leaching potential. Because the reductions in production costs due to reduced water applications were small and because the check treatment resulted in significant environmental benefits, there would be no benefit from deficit irrigation drier than the check treatment. In eastern Oregon, deficit irrigation after tuber set could lead to greater risk to potato growers and could reduce the processing industry's competitiveness due to deficiencies in tuber yield and quality.

Opportunities to conserve water through irrigation scheduling

In the present study, the leaching potential, as determined by the SWP treatments, was low, even for the wettest treatment. In each year, SWP at 0.5-m depth remained lower (drier) than at 0.2 m for all treatments, and total water applied (irrigation plus precipitation) was less or slightly less than the estimated ETc, suggesting that loss of water by leaching was minimal (Figure 3). Irrigation scheduling, using both a target SWP and controlled water application that did not exceed the water holding capacity of the top 0.3 m of soil, resulted in total seasonal water applied being slightly less than estimated ETc, even wirh irrigation at -60 kPa. Stored moisture at lower depths in the soil profile can in part supply this small water deficit, as suggested by the increasing dryness of the soil at 0.5 m for the check treatment in 1993 and 1994 (Figure 3). Alternatively, small water savings may accrue by limiting irrigations before tuber set (Shock et al., 1992). Where feasible, SDI can improve water use efficiency for potato compared to sprinkler irrigation, by reducing evaporative losses of water (DeTar et al., 1995; Sammis, 1980; Shea et al., 1999).

The ideal SWP for irrigation sche-duling varies from -20 to -60 kPa, depending on soil type, irrigation system, production area, and variety (Holder and Cary, 1984; van Loon, 1981). For silt loam in eastern Oregon, the soil water potential in the top 0.3 m should remain wetter than -60 kPa. Irrigation of Russet Burbank on sandy soils in Australia required -20 kPa during early tuber bulking and -20 to -40 kPa after-wards (Hegney and Hoffman, 1997). Careful irrigation scheduling with an appropriate local SWP irrigation criterion and ETc replacement can achieve efficient water use, while maintaining profitability in a crop sensitive to deficit irrigation.

References

Christiansen, J.E. 1942. Irrigation by sprinkling. California Agriculture Experiment Station Bulletin 670. Berkeley, California, United States of America, University of California.

DeTar, W.R., Browne, G.T., Phene, C.J. & Sanden, B.L. 1995. Real-time irrigation scheduling of potatoes with sprinkler and subsurface drip systems. In: Proceedings of the International Conference on Evapotranspiration and Irrigation Scheduling, San Antonio, St. Joseph, Michigan, United States of America, TX. American Society of Agricultural Engineers, .